Many electronic devices and systems include integrated circuits for the storage of data during the operation of the devices. For example, electronic devices such as computers, printing devices, scanning devices, personal digital assistants, calculators, computer work stations, audio and/or video devices, communications devices such as cellular telephones, and routers for packet switched networks may include memory in the form of integrated circuits for retaining data as part of their operation. Advantages of using integrated circuit memory compared to other forms of memory include space conservation and miniaturization, conserving limited battery resources, decreasing access time to data stored in the memory, and cutting the costs of assembling the electronic devices.
Dynamic Random Access Memory (“DRAM”) is an example of integrated circuit memory. DRAM typically comprises an array of semiconductor capacitor cells, each of which may hold an amount of electric charge that represents the logical value of a stored bit. The cells in the array are typically arranged in rows and columns. Each cell is situated at the intersection of a row and a column. Each cell in the DRAM array may be accessed by simultaneously addressing the intersecting row and column.
In operation, internal circuitry on the DRAM refreshes the charge on those cells that sense amplifiers have determined to already hold an electric charge. In this manner, the DRAM compensates for leakages of electric charge from the semiconductor capacitor cells, such as leakage into the substrate of the DRAM integrated circuit. Also, secondary sense amplifiers in the DRAM sense the amounts of electric charges stored on the capacitors. Based on the sensed electric charges, the outputs of the secondary sense amplifiers represent the logical values of the bits that are stored in the DRAM array. Data is written into the DRAM array and read out of the DRAM array through multiple read-write data lines. The read-write data lines connect the DRAM array to the input/output pins of the DRAM integrated circuit. The pins connect the DRAM integrated circuit to other integrated circuits in the electronic device. In this manner, the data stored in the DRAM array may be extracted from the DRAM integrated circuit for use by the other integrated circuits in the electronic device. Such reading, writing, and maintaining of charge on the cells are substantial internal operations of the DRAM.
Distinct sets of the read-write data lines typically connect to distinct portions of the DRAM memory array. Each of the memory portions also includes redundant memory cells that may substitute for damaged or failed memory cells within that memory portion. When all the redundant memory cells for a memory portion have been used as substitutes, however, further repair of the memory portion is not possible. Any additional damaged or failed memory cells in the memory portion cannot be repaired within that memory portion, resulting in corrupt or lost data on the read-write data lines and therefore a failed DRAM integrated circuit.
One possible solution to the problem is to increase the number of redundant memory cells in the memory portion. This solution, however, comes at the expense of an increase in the size of the die of the DRAM array as it has to accommodate many more memory cells. Therefore there is a need for repairing failed memory cells in a DRAM integrated circuit without increasing the size of the die of the DRAM array.